Molded records, such as conventional audio records and the more recently developed capacitive electronic discs, are commonly manufactured by compression molding of a thermoplastic composition. The thermoplastic compositions used to mold records are comprised of a thermoplastic resin such as polyvinyl chloride, polyvinyl acetate, polystyrene and the like and additives such as stabilizers, lubricants and antistatic agents and carbon black. In the compression molding process a preform is initially formed which contains an amount of a thermoplastic composition which is somewhat in excess of the amount of material actually needed to mold the record in order to insure that when the record is molded it will not have any voids and will be of uniform density.
The preform is placed in a record molding press between a pair of stampers which are initially heated to an elevated temperature above the melting point of the thermoplastic composition. The molding press is then closed which causes the heated stampers to both heat the thermoplastic composition of the perform above its melting point and to cause resulting molten thermoplastic composition to flow between the stampers and thereby form the record. During the molding process the excess amount of thermoplastic composition from the preform will flow past the outer edges of the stamper and will be formed into an irregular ring of material around the molded record which is referred to as flash. After the molten thermoplastic material has been pressed between the heated stampers, the stampers are cooled which in turn causes the thermoplastic composition to solidify in the form of a record.
After the molded record is solidified the flash which was formed on the outer diameter of the record must be removed and the outer diameter of the record trimmed to a specified final diameter. Various types of trimming apparatus which are generally well known in the art can be used to remove the flash. The type of trimmers which are typically employed include, for example, scissors cutters, knife cutters, routers and the like.
A serious production problem, referred to as breakout, is encountered with all of the commonly used types of trimmers. Breakout occurs in the trimming step and in the final finishing of records when the flash does not separate cleanly from the outer diameter of the record but rather a portion of the molded record is broken out along with the flash; or the edge of the record is cracked; or other similar defects occur, all of which causes destruction of the molded record.
While the exact cause of the breakout problem is not known for certain it is believed to be due in part to the relatively brittle nature of the solidified thermoplastic compositions used in the molding of records. This appears to be true particularly with regard to the thermoplastic composition used for molding capacitive electronic discs. Capacitive electronic discs must be electrically conductive. In order to obtain the required electrical conductivity the thermoplastic compositions used to mold the capacitive electronic discs are heavily loaded with conductive carbon which causes substantial embrittlement of the solidified thermoplastic composition.
It would be highly desirable to have an apparatus which would prevent breakout during removal of the flash and the final finishing of the outer diameter of thermoplastic records.